This invention provides a new approach to the problem of determining the focal parameters of optical elements or systems of the general type encountered in ophthalmic optics. Those parameters typically comprise the spherical power, the cylindrical power and the cylindrical axis or principal meridian.
In its more general aspects, the invention is useful for analyzing any optical system that may have appreciable cylindrical power and that is sufficiently compact axially to be usefully represented by a thin-lens approximation. In the particular field of ophthalmic optics the system under test may comprise the human eye itself or the lens or lens system by which errors of the eye are to be corrected.
Presently available procedures for measuring the focal parameters of such lens system are of two basic kinds. In one kind, a principal meridian is first determined, as by finding the meridian in which a linear target appears sharp. The focal power is measured in any suitable manner in that meridian and also in the second principal meridian, usually assumed to be at 90.degree. to the first. One of the resulting values represents the spherical power of the system, and their difference represents the cylindrical power.
Such prior art procedures have the disadvantage that the initial determination of a principal meridian tends to be time consuming and subject to appreciable error, especially when the cylindrical power is relatively small. Those difficulties are well described in the prior U.S. Pat. Nos. 3,785,723 and 3,841,760 to David L. Guyton, for example, together with special targets and associated procedures for aiding the operation.
In another kind of procedure for determining the ophthalmic parameters, a laser beam is nutated about the axis of a radially spoked target or reticle, which is then imaged by the optics under test. The imaging beam is sensed electronically and the signal is processed to compute the desired parameters, typically by taking the Fourier transform of the time domain signal and determining the frequency content by various applicable algorithms. Since that approach requires relatively sophisticated apparatus it is mainly adapted for fully automatic determination of the parameter values.
The present invention aims to solve the focusing problem in the time domain, avoiding any Fourier or like transformation and the accompanying technical uncertainties. At the same time, the invention retains the basic simplicity of the more conventional prior art, but avoids its primary problem by completly eliminating any initial step of locating a principal meridian of the system under test.